Poultry meat and eggs are important food sources, whose consumption increases continually due to the growth of the human population and their great quality-price ratio. The recent epidemic of avian influenza focused the public opinion on poultry health as well as food safety and security. Poultry vaccine technology became a worldwide concern.
Recombinant viruses expressing pathogen proteins are commonly used as poultry vaccines against targeted pathogens. Vaccines including such viruses induce expression of foreign pathogen proteins or fragments thereof within infected cells, which can subsequently induce a specific and protective humoral immunity as well as cell-mediated immunity.
It is known that different viruses can survive in the body of an infected animal in the state of latent or persistent infection. Consequently, such viruses, in which a foreign gene derived from a pathogen has been integrated, have been developed to be used as viral-vectored vaccines increasing the duration of immunity to an immunized animal. These viral vectors (or recombinant viruses) are based typically on avipox viruses, such as fowlpox (EP-A-0,517,292), herpes viruses, particularly HVT (e.g., WO-A-87/04463, U.S. Pat. Nos. 5,980,906, 5,853,733), Newcastle disease virus (NDV) or avian adenoviruses. These recombinant avian viruses display variable levels of protection. In particular, because Poxviruses, NDV, and adenoviruses do not persist in chickens, long duration of immunity is not expected. A recombinant HVT expressing IBDV VP2 has shown advantages over classical IBD vaccines (Vectormune® IBD). Other HVT vectors of interest express NDV (Vectormune® ND) or ILTV (Vectormune® LT) antigens.
One of the practical problems of HVT-based recombinant viruses is their interference when several viruses are used in combination to confer immunogenicity against distinct pathogens. Indeed, when two distinct rHVT expressing different antigens are mixed, a lower protection is caused at least against one of the disease (see e.g., Slacum G et al., 2009, The compatibility of HVT recombinants with other Marek's disease vaccines, 58th Western Poultry Disease Conference, Sacramento, Calif., USA, March 23-25, p 84).
Multivalent HVT vectors have been developed which can express two distinct antigenic peptides (see PCT/EP2013/056839) and potentially overcome the limitations of the prior art. Also, new viral serotypes are being explored, with the aim to find alternative compatible viral vectors. In this regard, MDV1 has been experimentally used but the recombinants generated so far did not provide satisfactory results.
Accordingly, there is still a need for new alternative approaches to improve vaccination in animals, particularly in poultry, allowing stable protein expression and, preferably concomitant protection against several diseases.